1. Field of the Invention
The present invention relates to gravity meters in general and in particular to a gravity meter having an improved nulling and measuring system.
2. Background of the Invention
Gravity meters have long been used by geophysicists and others interested in the bulk in-situ measurement of rock densities. Most gravity meters utilize a weight disposed at one end of a horizontal weight arm with the other end of the arm being secured to a pivotal support. The weight arm is maintained at a desired reference position by a mainspring which is often secured to the end of a second support arm mounted to the frame of the instrument. The mainspring is selected to counteract the force of gravity acting on the weight arm over a specific range of gravity, and a mechanical adjustment system is usually attached to the support arm to balance the weight arm to a desired reference or null point. Once the gravity meter is hulled, a change in the ambient gravitational field between two observation stations, or a change in the ambient gravitational field at the same station over time, causes a displacement of the weight arm. The displacement of the weight arm may be measured and used to calculate the change in the gravitational field. In short, then, the gravity meter operates on the principle of balancing the force of gravity by varying the force applied by the spring to maintain the weight arm at the reference or null point.
A significant problem that confronts the use of such gravity meters arises as a result of the world-wide variance in the earth's gravitational field. As noted above, gravity meters are designed to operate within ranges determined by the characteristics of their mainsprings. The problem is that no mainspring has yet been capable of accurately responding to the full range of gravity variance encountered throughout the world while at the same time having suitable sensitivity for bore hole gravity measurements. Therefore, it is necessary to select a meter for use in each local area according to the gravity field range in that area. Users of these meters must, therefore, have several different meters to cover the complete range of gravity variance likely to be encountered.
Another significant problem confronting such gravity meters is the need to provide a nulling and measuring system that will accurately reflect movement of the reaction mass mounted on the weight arm, so that the changes in the ambient gravitational field can be calculated from the magnitude of that movement. For example, a gravity meter has been developed that has a readout system based on the position of a metal weight arm between two conductor plates. A square wave signal is placed on each plate with the signal normally being 180.degree. out of phase. The position of the weight arm can be thus affected by imposing a direct current signal on a selected one of the plates while the position of the weight arm is determinable from monitoring the resultant signal generated on the weight arm from the two square wave signals.
As another example, a previous patent (U.S. Pat. No. 4,422,329), issued to one of the co-inventors herein, teaches an improved gravity meter having a novel support and suspension system for the weight arm and a unique hulling and sensor system to allow remote hulling of the gravity meter and the remote sensing of the change in the gravitational field. The nulling and measuring system of that invention utilizes a two plate conductor and dielectric system, which balances the torque force due to gravity with the combined force of the mainspring and the body force on the dielectric. Unfortunately, in that system, the body force on the dielectric is proportional to the square of the voltage imposed on the plates, which leads to difficulties in measuring the nulling force and can reduce the overall accuracy of the system. Moreover, that system is not capable of measuring directly the position of the weight arm, but instead relies on detecting the change in the capacitance of the two conductor plates. Therefore, if the weight arm of that system is against the limit stop, the operator has no way of knowing whether the lack of change in capacitance is attributable to an equilibrated condition wherein the forces are in balance, or whether the lack of change in capacitance is due to the weight arm being against the limit stop. Finally, while that system does provide a means for determining the absolute capacitance of the conductor plates, so the operator can determine whether the weight arm is against the limit stop, such absolute capacitance measurement is difficult, and drift and static build-up on the plates make such measurements unreliable at best.